The present invention relates generally to a graphics processing apparatus, and more particularly to a graphics processing apparatus in which an improved antialiasing process is carried out at a high speed to reduce the effects of aliasing from edge portions of an output image so that smooth edge portions of the output image are created.
Conventionally, in the computer graphics field, an antialiasing technique is used when an image with aliasing is outputted to a display unit such as a CRT display, so that a better display image with smooth edges is created. There are several prior antialiasing techniques for reducing the effects of aliasing from edge portions of an output image to create an image with smooth edges, and these prior techniques include the averaging technique, the filtering technique and the prefiltering technique. By carrying out a luminance modulation in such a prior antialiasing technique, a stair-like irregularity or aliasing at an edge portion of the image as shown in FIG. 1(a) is eliminated so that a visually smooth edge portion of the image as shown in FIG. 1(b) is created.
In a case in which the averaging technique is used, each pixel is divided into N.times.M subpixels (N, M: a natural number) and the raster computing of a high-resolution image is carried out, then averaging the N.times.M subpixels for low resolution to determine a luminance of each pixel. A more detailed description of the averaging technique for antialiasing will now be given with reference to FIG. 2(a), (b). When a pixel being considered lies at an edge portion of an image (the image shown in FIG. 2 extends continuously toward the left hand direction of the inclined line). When the antialiasing process is not carried out, the maximum intensity available (luminance, or luminous intensity, "kid"=255, in a case of 256-level tone image) is assigned to this edge-portion pixel as shown in FIG. 2(a). When an antialiasing process is carried out by the averaging technique, the edge-portion pixel is divided into, for example, 7.times.7 subpixels as shown in FIG. 2(b). The number of subpixels which are located inside the image is counted, and in this case the counted number of such subpixels is 28. This counted number is divided by the total number (in this case, the total number (=49) of subpixels within the pixel being considered for averaging the luminous intensity of the subpixels, and the maximum intensity of 255 is multiplied by 28/49 to determine a suitable luminous intensity to be assigned to each subpixel. In this manner, when the averaging technique is used, the luminous intensity of a pixel is thus determined by considering where the pixel being considered is located in the image.
The filtering technique is a modified method of the averaging technique described above. In the averaging technique, the same weight value is assigned to all the subpixels within one pixel. In the filtering technique, however, a different weight value (which is called hereinafter a weight) is assigned the respective subpixels by using a prescribed filter. The intensity of a subpixel is influenced by a factor which is determined depending on the location of the subpixel in the image. Referring to FIG. 3(a), (b), an example of the filtering technique is now considered, and in this example the filtering technique is applied to the same image data as shown in FIG. 2(a) and the pixel being considered is divided into 7.times.7 subpixels. FIG. 3(a) shows the character data of the so-called cone filter, which is used to assign a weight given from the character data of the cone filter shown to the corresponding subpixels. For example, a weight of 2 is assigned to the right corner subpixel to determine the intensity of the subpixel. When a number of subpixels are located within the image, the counted number of the subpixels is determined by a weight from among the character data of the cone filter. In FIG. 2(b), a pattern of the display image after the antialiasing process is carried out by the filtering technique is shown. In this case, the counted number of subpixels with weighted intensities which are located in the image is 199, and this number of 199 is divided by the total number (366) of the weights in the cone filter for averaging the subpixels, and the maximum intensity is multiplied by 199/366 to determine a suitable intensity to be assigned to the pixel. And, there are several known filters which have been used in the filtering technique as described above, and some of them are illustrated in FIGS. 4(A-D).
The prefiltering technique is an antialiasing method in which consideration is also given to the intensity of neighboring pixels of a pixel when the intensity of that pixel is determined. In other words, an antialiasing process is carried out by applying the averaging technique or the filtering technique to N'.times.N' pixels around a pixel being considered. Referring now to FIG. 5, a case in which the prefiltering technique is used for carrying out an antialiasing process of 3.times.3 pixels is described. In FIG. 5, a pixel the intensity of which is determined by this method is indicated by 6201. The image extends continuously toward the lower right side of FIG. 5, the number of subpixels, indicated in black, which are located in the image is counted. Each pixel is divided into 4.times.4 subpixels. A filter used in this case is formed by a 12.times.12 matrix. The prefiltering technique has also the effect of eliminating high frequency components from a vector based image data.
With the recent development of the so-called desktop publishing (DTP) system employing a personal computer, a DTP system which can perform a printing of a vector based image data such as a computer graphics data has widely been used in the field. A typical DTP system is the PostScript system from Adobe Co., and the PostScript is a kind of a programming language which is generally known as the Page Description Language (PDL). This programming language is used to describe the contents of an image or document for each one page and define the arrangement of the page, including the style and format of a text region as well as a graphic region within the page. In the DTP systems of this kind, a vector font is used instead of a conventional character font frequently used in the prior publishing system, and therefore such a vector image processing system can provide a superior image quality of an output image when compared with the image quality of an image outputted by the conventional system such as a wordprocessor system using the so-called bit map font. And, this vector image processing system has an advantage in that a composite image in which a character font data and a graphics data are included in a mixed manner can be outputted.
However, a laser printer having a relatively low resolution of 200 dpi to 400 dpi is usually used with the above described conventional system, and there is a problem in that aliasing phenomenon often takes place in a printed image outputted by the laser printer, especially in the case of an output image, such a computer graphics image. Therefore, there exists a demand to create an increased image quality of an output image outputted by the laser printer through an improved antialiasing process to suitably reduce the effects of aliasing from the output image. However, in the case of the graphics processing apparatus to which the prior antialiasing process is applied, computing the area factor or the intensity of a pixel is a complicated job and takes much time. It is difficult for such a conventional apparatus to carry out at a high speed the antialiasing and image displaying processes, and there is a problem in that an increase of a graphics processing speed cannot be attained with ease because the conventional antialiasing process is used. Especially, in the case in which the prefiltering technique is used, many computing steps are required and many subpixels are influenced by the computation, and it is very difficult to attain a faster graphics processing.